The present invention relates generally to electrical switching devices, and more particularly, to a method and apparatus for monitoring the wellness of contactors and motor starters, especially electromagnetic contactors and motor starters. The present invention measures various currents and voltages in one or both of the switched line and the actuating coil to monitor performance and determine indications of impending faults of the device.
Contactors are generally used in motor starter applications to switch on/off a load as well as to protect a load, such as a motor, or other electrical devices from current overloading. As such, a typical contactor has three contact assemblies—a contact assembly for each phase or pole of a three-phase electrical device. Each contact assembly, in turn, includes a pair of stationary contacts and a pair of moveable contacts. One stationary contact will be a line side contact and the other stationary contact will be a load side contact. The moveable contacts are controlled by an actuating assembly comprising a contact carrier and an armature magnet assembly which is energized by a coil to move the moveable contacts to form a bridge between the stationary contacts. When the moveable contacts are engaged with both stationary contacts, current is allowed to travel from the power source or line to the load or electrical device. When the moveable contact is separated from the stationary contacts, an open circuit is created and the line and load are electrically isolated from one another.
Each contact assembly, and each set of moveable and stationary contacts thereof, corresponds to a pole or phase of the same three phase input. Thus, in some contactors, the three pairs of moveable contacts are all moved between open and closed positions in unison. Other contactors, however, provide for independent or timed control of each pair of moveable contacts, such as in systems that use so-called “Point-on-Wave” switching. In addition, many contactors utilize variations intended to render them more tolerable or more sensitive to current overloads, such as contacts that automatically blow open upon an overload before an open command is received. The development of these alternatives illustrates a general recognition in the art that, despite their relative durability, all contactors have a finite useable life. Component wear, contact surface erosion, friction, jam, contact welding, arc-generated debris, and other factors limit the length of time and/or number of operations through which a contactor may be used.
Since contactors and motor starters are important components of both automation and control systems, monitoring their remaining useable life, or “wellness,” to predict impending faults before occurrence is essential. Un-predicted failures of contactors not only cause costly work stoppages, but also can cause damage to the load and other related systems and equipment. In contrast, over-cautious approaches to contactor monitoring and replacement increase maintenance costs and slow or delay usage of the motor/load.
Currently, most methods for estimating the working life of contactors rely upon the manufacturer's life test data or guidelines. That is, most commercially available contactors have a designated number of operations or cycles after which the manufacturer recommends replacement to avoid failure in use. Thus, many systems and methods for predicting failure simply count the number of operations that a contactor completes. However, each contactor will not necessarily operate for the same number of cycles before failure. And, the causes of failure vary among contactors as well as the conditions which lead to possible failure issues. How a contactor is operated, the conditions under which it is used, and the characteristics of the environment in which it is used cause even more variation in the number of operations a contactor might undergo before failure. Therefore, to be useful, counting methods must be overly cautious in setting replacement schedules, or risk contactor failures while in use.
Other approaches for monitoring contactors have been centered on determining whether a connection between the movable and stationary contacts was actually made properly. Thus, some systems have compared actuating coil current to reference values to determine whether contacts have fully closed. Similar systems have measured the impedance of the actuating coil by monitoring the decay rate of current therethrough during a period when a supply regulator is turned off. Since impedance will vary appreciably depending on whether the contacts are fully open or closed, the state of the contacts can be determined. More simplistic methods of monitoring contactors have involved the use of simple mechanical translations of the position of the contacts, whether open or closed. Other approaches use optical devices to detect the presence or brightness of arc emissions indicating that a failure has occurred. However, such approaches are not believed to have the ability to reliably predict impending failures, only to detect existing failures.
Systems similar to those described above are also used for safety interlocking. That is, an additional set of contacts are coupled to the primary moveable and stationary contacts such that they engage in a closed position when the primary contacts engage and separate when the primary contacts separate. These additional sets of contacts are known as interlocks or mirror contacts. The drawback to such a method of ensuring proper contact closure is that only a rough mechanical translation of contact closure is available. Thus, the interlock contacts are just as susceptible to jam, friction, wear, erosion, and other problems as are the primary contacts. Also, even when working properly, the interlocks provide limited information—whether the contacts are properly closed. In contrast, a system which predictively monitors currents and/or voltages of the electromagnetic contactor itself can provide more data on contact movement and can provide such data throughout a complete operating cycle (initiation through coil operation and current flow to contact opening).
Drawbacks of the above methods are that they cannot accurately predict failures (they detect failures), they require additional costly hardware, they require add-ons that are bulky and not durable, they use components susceptible to damage from contact arcing, they waste contacts which have significant remaining life, and worst, they are unreliable for indicating contactor wellness and predicting impending faults.
It would therefore be desirable to have a system and method capable of accurately monitoring the remaining useable life of a contactor and impending faults thereof. Preferably, such system should not rely upon manufacturer recommended operation counts and should predict rather than merely detect failures. In addition, it would be desirable if such a system could also perform safety interlock or mirror contact functions.